As was mentioned above and is discussed in more detail below, the invention concerns an induced polarization (IP) detector. The IP effect is a current-induced electrical response detected as a delayed voltage in certain minerals and, as described below, the method has been used for some time in the detection of these minerals in the ground. One manifestation of the response is that the voltage on an array of detectors or receivers lags the primary or inducing voltage (produced by a transmitter) by a finite amount of time. This is usually expressed as a phase-shift, i.e., a slight shift of the wave-cycle between the transmitter and receiver, and is usually reported in units of milliradians, where one duty cycle of the transmitter is 2.pi. radians.
For many decades it has been known that pyrite, most other metallic-luster minerals, and certain clays give rise to an IP effect. Geophysicists have taken advantage of this fact to discover and map large disseminated sulfide bodies (primarily copper and molybdenum) since the 1950's. The phenomenon is based on a complex double-layer interaction of ions in the electrolyte (the ground water) and the individual mineral surfaces. Because of this, IP is more sensitive to surface area than to volume and finely disseminated minerals make the best targets. An IP survey typically gathers both resistivity information, which is generally a measure of the porosity of the substrate, and polarization information, which is a measure of the reactivity of certain minerals (i.e., those described above) disseminated throughout the surface. Computer modeling can then be used to arrive at models that best fit the observed data acquired on the surface, with the purpose of providing a true map of the three-dimensional nature of the subsurface. The use of two physical characteristics (resistivity and polarization information) instead of just one makes the interpretation much more reliable.
There are, of course, a number of patents relating to induced polarization detectors for the detection of minerals and other materials, and among these are the following: U.S. Pat. No. 5,671,136 (Willhoit), U.S. Pat. No. 4,467,642 (Givens), U.S. Pat. No. 4,041,372 (Miller, et al.), and U.S. Pat. No. 3,984,759 (St. Amant, et al.). These patents are not concerned with detectors adapted for use in a subsea environment.
It will be appreciated that the use of IP detection in a subsea environment presents tremendous engineering obstacles, including current channeling in the seawater and noise problems because of the low signal-to-noise ratio of the signals involved. Because of these and other obstacles, to the knowledge of applicant, IP detection systems have not used previously in the detection of minerals and the like on the sea floor.